Alkali-free glass substrate

ABSTRACT

In order to provide an alkali-free glass substrate which contains substantially no alkali metal oxide, which has chemical resistance, a high strain point, excellent meltability, and devitrification resistance, the alkali-free glass substrate contains substantially no alkali metal oxide and essentially consists, by weight, of 58.0-68.0% SiO 2 , 10.0-25.0% Al 2  O 3 , 3.0-15.0% B 2  O 3 , 0-2.9% MgO, 0-8.0% CaO, 0.1-5.0% BaO, 0.1-10.0% SrO, 0-5.0% ZnO, 0-5.0% ZrO 2 , and 0-5.0% TiO 2 .

CROSS-REFERENCE TO RELATED APPLICATION

This application is a 371 of international application No. PCT/JP96/02751, filed Sep. 25, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an alkali-free glass substrate for use as a substrate for a display such as a liquid crystal display and an EL display, a filter, a sensor, and the like.

2. Description of Prior Art

Conventionally, a glass substrate has widely been used as a substrate for a flat panel display such as a liquid crystal display, a filter, a sensor, and the like.

On a surface of the glass substrate of the type, a transparent conductive film, an insulating film, a semiconductor film, and a metal film are deposited and various circuits or patterns are formed by photolithography etching (photo etching). In such deposition and photo etching processes, the glass substrate is subjected to various kinds of heat treatment and chemical treatment.

For example, in case of an active matrix liquid crystal display of a thin-film-transistor (TFT) type, the insulating film and the transparent conductive film are deposited on the glass substrate and a number of TFTs of amorphous silicon or polycrystalline silicon are formed by the photo etching. In these processes, the glass substrate is subjected to the heat treatment at a temperature on the order of several hundreds degrees centigrate and to the chemical treatment by various chemicals such as sulfuric acid, hydrochloric acid, alkali solution, hydrofluoric acid, and buffered hydrofluoric acid.

Among others, buffered hydrofluoric acid is widely used in etching of the insulating film but tends to erode a glass so that its surface is clouded. Furthermore, reaction with a glass component occurs and produces a reaction product, which may clog a mesh or pores of filtration screens used in the process or adhere onto the glass substrate.

On the other hand, hydrochloric acid is used in etching of an ITO film and a chromium film but tends to erode the glass so that its surface suffers discoloration, cloudness, and cracks. It is very important to provide buffered-hydrofluoric-acid resistance and hydrochloric-acid resistance to the glass substrate of the type.

Therefore, the glass substrate for use in the active matrix liquid crystal display of a TFT type is required to have following characteristics.

(1) To contain substantially no alkali metal oxide because, if alkali metal oxide is contained in the glass, alkali ion is diffused in a deposited semiconductor material during heat treatment to result in degradation of a film characteristic.

(2) To have chemical resistance such that no degradation is caused by chemicals including various kinds of acid and alkali used in a photo etching process.

(3) To cause no heat contraction by heat treatment in a deposition or an annealing process. For this purpose, to have a high strain point. For example, in case of a polycrystalline silicon TFT-LCD requiring a working temperature of about 600° C. or more, the glass substrate for such application must have a strain point of 650° C. or more.

Taking the meltability and the formability into consideration, the glass substrate of the type is further required to have following characteristics.

(4) To be excellent in meltability so that a melting defect unfavorable as the glass substrate does not occur in the glass.

(5) To be excellent in devitrification resistance so that no foreign substance is produced in the glass during melting or forming.

In recent years, an electronic device such as the active matrix liquid crystal display of a TFT type is coming into use in a field of personal application and is therefore required to be light in weight. Consequently, the glass substrate is also required to be light in weight so that reduction in thickness is pursued. On the other hand, the electronic device of the type is also increased in size. Under the circumstances, the reduction in thickness is inevitably limited, taking the strength of the glass into consideration. Therefore, it is required to reduce the density of the glass in order to make the glass substrate light in weight.

As the substrate for the active matrix liquid crystal display of a TFT type, use has traditionally been made of alkali-free glass materials, such as quarz glass, barium borosilicate glass, and aluminosilicate glass, each of which has both advantages and disadvantages, however.

Specifically, the quarz glass is excellent in chemical resistance and heat resistance and has a low density but requires a high material cost.

As the barium borosilicate glass, there is a commercially-available product #7059 supplied by Corning. However, this glass is inferior in acid resistance so that transformation, cloudness, and roughness are easily caused on the surface of the glass substrate. In addition, an eluting component from the substrate tends to contaminate a chemical solution. Furthermore, this glass has a low strain point and is readily subjected to heat contraction and heat deformation so that heat resistance is insufficient. The density of the glass is as high as 2.76 g/cm³.

On the other hand, the aluminosilicate glass is excellent in heat resistance. However, most of commercially-available glass substrates are inferior in meltability and unsuitable for mass production. In addition, most of these glass substrates have a high density of 2.7 g/cm³ or more, and are inferior in buffered-hydrofluoric-acid resistance so that, at present, there is no glass substrate satisfying all of the required characteristics.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an alkali-free glass substrate which satisfies all of the above-mentioned required characteristics (1) to (5) and which has a density of 2.55 g/cm³ or less.

An alkali-free glass substrate according to the present invention essentially consists, by weight, of 58.0-68.0% SiO₂, 10.0-25.0% Al₂ O₃, 3.0-15.0% B₂ O₃, 0-2.9% MgO, 0-8.0% CaO, 0.1-5.0% BaO, 0.1-10.0% SrO, 0-5.0% ZnO, 0-5.0% ZrO₂, and 0-5.0% TiO₂, and contains substantially no alkali metal oxide.

At first, description will be made about the reason why the components of the alkali-free glass substrate of this invention are defined as mentioned above.

SiO₂ is a component serving as a network former of the glass. With the increase in content of SiO₂, the density is easily lowered down to 2.55 g/cm³ or less. Therefore, in this invention, the content is selected to be 58.0% or more. However, when the content is more than 68.0%, the high-temperature viscosity is increased so that the meltability is deteriorated. In addition, the devitrification is seriously deteriorated so that the devitrified foreign substance of cristobalite tends to precipitate in the glass. Thus, the content of SiO₂ is preferably 58.5-67.0%.

Al₂ O₃ is an essential component required to improve the heat resistance and devitrification resistance of the glass and to lower the density. The content of Al₂ O₃ is 10.0-25.0%, preferably, 15.0-23.0%. When the content is less than 10.0%, the tendency of devitrification is increased so that the devitrified foreign substance of cristobalite may precipitate in the glass. In addition, the strain point becomes low. When the content is more than 25.0%, buffered-hydro-fluoric-acid resistance is degraded so that the cloudness is readily caused to occur on the surface of the glass substrate, and the high-temperature viscosity of the glass is increased and the meltability is deteriorated.

B₂ O₃ is a component serving as a melting agent to lower the viscosity and to improve the meltability. The content of B₂ O₃ is 3.0-15.0%, preferably, 6.5-15.0%, and more preferably, 8.5-15.0%. When the content is less than 3.0%, the function as the melting agent is insufficient and the buffered-hydrofluoric-acid resistance is degraded. When the content is more than 15.0%, the strain point of the glass is lowered so that the heat resistance is degraded. In addition, the acid resistance is also degraded.

MgO is a component for decreasing the high-temperature viscosity without lowering the strain point so as to improve the meltability of the glass. Furthermore, MgO is a component which has a greatest effect of reducing the density among bivalent alkaline earth metal oxides. However, a large content of MgO is unfavorable because the tendency of devitrification is increased. Therefore, the content of MgO is 0-2.9%, preferably, 0-1%.

Like MgO, CaO is a component having a function of decreasing the high-temperature viscosity without lowering the strain point so as to improve the meltability of the glass. The content of CaO is 0-8.0%, preferably, 1.8-7.5%, more preferably, 2.1-7.5%. The content more than 8.0% is unfavorable because the buffered-hydrofluoric-acid resistance of the glass is seriously degraded. Specifically, when the glass is subjected to buffered hydrofluoric acid treatment, a large amount of a reaction product by a CaO component in the glass and the buffered hydrofluoric acid precipitates on the surface of the glass so that the glass substrate is readily clouded. In addition, those elements formed on the glass substrate and the chemical solution may often be contaminated by the reaction product.

BaO is a component to improve the chemical resistance and the devitrification resistance of the glass. The content of BaO is 0.1-5.0%, preferably, 0.1-4.5%. When the content is less than 0.1%, it is difficult to obtain the above-described effect. The content more than 5.0% is unfavorable because the density of the glass is increased.

Like BaO, SrO is a component to enhance the chemical resistance of the glass and to improve the devitrification resistance. Furthermore, SrO has a characteristic such that the meltability is hardly deteriorated, in comparison with BaO. The large content of SrO is unfavorable because the density of the glass is increased. Therefore, the content of SrO is 0.1-10.0%, preferably, 1.0-9.0%.

ZnO is a component to improve the buffered-hydrofluoric-acid resistance and to improve the meltability. The content of ZnO is 0-5.0%. When the content is more than 5.0%, the glass tends to be devitrified. In addition, the strain point is lowered so that excellent heat resistance can not be achieved.

It is noted here that, when the total content of MgO, CaO, SrO, BaO, and ZnO is less than 5.0%, the viscosity at a high temperature is increased so that the meltability is degraded and the glass is readily devitrified. On the other hand, the total content of MgO, CaO, SrO, BaO, and ZnO more than 20.0% is unfavorable because the density of the glass is increased.

ZrO₂ is a component to improve the chemical resistance, particularly, the heat resistance of the glass and to improve the meltability by reducing the high-temperature viscosity. The content of ZrO₂ is 0-5.0%, preferably, 0.1-4.0%. When the content is more than 5.0%, the devitrification temperature rises so that the devitrified substance of zircon readily precipitates.

TiO₂ is also a component to improve the chemical resistance, particularly, the acid resistance, to lower the high-temperature viscosity so as to improve the meltability, and to avoid coloring by ultraviolet rays. Specifically, when a liquid crystal display is manufactured, the ultraviolet rays are sometimes irradiated in order to remove an organic substance on the glass substrate. However, coloring of the glass substrate by the ultraviolet rays is unfavorable because the transmittance is degraded. Therefore, the glass substrate of the type is required not to be colored by the ultraviolet rays. However, the content of TiO₂ more than 5.0% is unfavorable because the glass tends to be colored.

In the present invention, it is possible to add other components than the above-described ones as far as the characteristics are not deteriorated. For example, it is possible to add as a clarifying agent those components such as As₂ O₃, Sb₂ O₃, F₂, C12, SO₃, and SnO₂ as well as metal powder such as Al and Si.

However, it is unfavorable that alkali metal oxide is contained in the glass because the characteristics of various films or semiconductor elements formed on the glass substrate are deteriorated.

PbO typically used as the melting agent seriously degrades the chemical resistance of the glass. In addition, PbO is unfavorable because it may volatilize from the surface of the melt during melting to cause environmental pollution.

P₂ O is also generally used as the melting agent. However, P₂ O₅ is unfavorable because the glass is phase-separated and the chemical resistance is seriously degraded.

When CuO is contained, the glass is colored and can not be used as a glass substrate for a display.

DESCRIPTION OF PREFERRED EMBODIMENTS

Now, the alkali-free glass substrate according to the present invention will be described more in detail in conjunction with specific examples.

Tables 1 to 3 show glass samples (samples Nos. 1-10) according to the present invention and comparative glass samples (samples Nos. 11-14).

Each of the samples shown in Tables was prepared as follows. At first, a glass material was prepared to have a composition shown in the Tables. The glass material was put in a platinum crucible and melted at -1580° C. for 24 hours. The glass material being melted was poured on a carbon plate and formed into a plate shape.

As is obvious from Tables, each of the samples Nos. 1-10 of the present invention had a density of 2.51 g/cm³ or less and a strain point of 668° C. or more. The samples Nos. 1-10 were excellent in hydrochloric-acid resistance, buffered-hydrofluoric-acid resistance, and devitrification resistance. Furthermore, in each of the samples Nos. 1-10, the temperature corresponding to 10².5 poises was 1625° C. or less. Thus, each of the samples Nos. 1-10 of the present invention exhibited an excellent characteristic.

                                      TABLE 1                                      __________________________________________________________________________                                         (wt %)                                             Sample No.                                                                     This Invention                                                         Composition                                                                            1   2   3   4   5   6   7   8                                          __________________________________________________________________________     SiO.sub.2                                                                              60.0                                                                               61.0                                                                               59.9                                                                               62.5                                                                               64.0                                                                               61.5                                                                               61.0                                                                               63.0                                       Al.sub.2 O.sub.3                                                                       17.0                                                                               18.0                                                                               16.5                                                                               20.5                                                                               21.0                                                                               19.0                                                                               13.5                                                                               17.0                                       B.sub.2 O.sub.3                                                                        9.0 11.0                                                                               9.0 6.0 6.5 7.5 8.5 10.0                                       MgO     --  0.5 --  1.5 0.5 1.0 --  --                                         CaO     5.5 3.5 2.1 6.0 7.0 3.0 2.5 7.5                                        BaO     4.0 1.0 3.5 1.5 0.5 0.5 2.0 0.5                                        SrO     3.5 2.0 6.5 1.5 0.5 5.0 9.0 1.0                                        ZnO     0.5 1.0 0.5 --  --  1.0 --  1.0                                        ZrO.sub.2                                                                              0.5 2.0 1.0 --  --  --  1.5 --                                         TiO.sub.2                                                                              --  --  1.0 0.5 --  1.5 2.0 --                                         Density (g/cm.sup.3)                                                                   2.50                                                                               2.40                                                                               2.51                                                                               2.45                                                                               2.39                                                                               2.46                                                                               2.51                                                                               2.39                                       Strain Point (°C.)                                                              681 689 676 691 719 671 668 670                                        Hydrochloric-acid                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                              Resistance                                                                     Buffered-hydro-                                                                        ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                              fluoric-acid                                                                   Resistance                                                                     Devitrification                                                                        ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                              Resistance                                                                     10.sup.2.5 Poise                                                                       1592                                                                               1611                                                                               1604                                                                               1621                                                                               1623                                                                               1625                                                                               1605                                                                               1594                                       Temperature (°C.)                                                       __________________________________________________________________________

                  TABLE 2                                                          ______________________________________                                                                    (wt %)                                                             Sample No.                                                                     This                                                                           Invention                                                       Composition      9       10                                                    ______________________________________                                         SiO.sub.2        63.5    61.5                                                  Al.sub.2 O.sub.3 19.0    18.5                                                  B.sub.2 O.sub.3  8.5     9.0                                                   MgO              0.2     --                                                    CaO              6.8     3.0                                                   BaO              0.5     0.5                                                   SrO              1.0     5.0                                                   ZnO              --      1.0                                                   ZrO.sub.2        0.5     0.5                                                   TiO2             --      1.0                                                   Density (g/cm.sup.3)                                                                            2.39    2.45                                                  Strain Point (°C.)                                                                       701     668                                                   Hydrochloric-acid                                                                               ◯                                                                          ◯                                         Resistance                                                                     Buffered-hydro-  ◯                                                                          ◯                                         fluoric-acid                                                                   Resistance                                                                     Devitrification  ◯                                                                          ◯                                         Resistance                                                                     10.sup.2.5 Poise 1598    1625                                                  Temperature (°C.)                                                       ______________________________________                                    

                  TABLE 3                                                          ______________________________________                                                                   (wt %)                                                          Sample No.                                                                     Comparative Example                                                 Composition  11      12        13    14                                        ______________________________________                                         SiO.sub.2    61.0    62.5      61.0  69.0                                      Al.sub.2 O.sub.3                                                                            13.0    18.5      15.0  11.5                                      B.sub.2 O.sub.3                                                                             9.5     6.5       5.0   5.5                                       MgO          5.0     2.0       2.5   1.0                                       CaO          4.5     6.5       3.0   4.0                                       BaO          4.0     --        7.0   4.0                                       SrO          2.0     4.0       5.0   3.0                                       ZnO          1.0     --        1.5   2.0                                       ZrO.sub.2    --      --        --    --                                        TiO.sub.2    --      --        --    --                                        Density (g/cm.sup.3)                                                                        2.54    2.47      2.63  2.50                                      Strain Point (°C.)                                                                   650     682       697   660                                       Hydrochloric-acid                                                                           ◯                                                                          Δ   ◯                                                                        ◯                             Resistance                                                                     Buffered-hydro-                                                                             ◯                                                                          ×   ◯                                                                        ◯                             fluoric-acid                                                                   Resistance                                                                     Devitrification                                                                             × ×   ◯                                                                        ×                                   Resistance                                                                     10.sup.2.5 Poise                                                                            1570    1507      1620  1705                                      Temperature (°C.)                                                       ______________________________________                                    

On the other hand, the comparative sample No. 11 was inferior in devitrification resistance. The sample No. 12 was inferior in chemical resistance and devitrification resistance. The sample No. 13 had a high density. The sample No. 14 was inferior in devitrification resistance, and had a high temperature corresponding to 10².5 poises so that the meltability is poor.

The density in Tables was measured by a well known Archimedes' method. The strain point was measured based on the ASTM C336-71 method.

The hydrochloric-acid resistance was evaluated by observing the surface condition of the glass substrate after each sample was optically polished and immersed in a 10 wt % hydrochloric acid solution held at 80° C. for 24 hours. The buffered-hydrofluoric-acid resistance was evaluated by observing the surface condition of the glass substrate after each sample was optically polished and immersed in a buffered hydrofluoric acid solution comprising 38.7 wt % ammonium fluoride and 1.6 wt % hydrofluoric acid held at 20° C. for 30 minutes. The symbols, X, Δ, and ◯ represent presence of cloudness or cracks on the surface of the glass substrate, presence of a little cloudness, and no change, respectively.

The devitrification resistance was evaluated by preparing, from each sample, glass powder having a particle size of 300-500 μm, putting the glass powder into a platinum boat for heat treatment at 1200° C. for 100 hours, and observing devitrification. The symbols X and ◯ represent presence of even a little devitrification and no devitrification at all, respectively.

10².5 poise temperature represents the temperature corresponding to 10².5 poises as the high-temperature viscosity. The lower this temperature is, the more excellent the melting formability is.

As described above, according to the present invention, there is provided the alkali-free glass substrate which contains substantially no alkali metal oxide, which is excellent in heat resistance, chemical resistance, and melting formability, and which has a low density of 2.55 g/cm³ or less.

Industrial Applicability:

As described above, the alkali-free glass substrate according to the present invention can be used as a substrate for a display such as a liquid crystal display and an EL display, a filter, a sensor, and the like, and is particularly suitable as a glass substrate for use in a TFT-type active matrix liquid crystal display required to be light in weight. 

I claim:
 1. An alkali-free glass substrate consisting essentially of, by weight, 58.0-68.0% SiO₂, 10.0-25.0% Al₂ O₃, 3.0-15.0% B₂ O₃, 0-2.9% MgO, 0-8.0% CaO, 0.1-5.0% BaO, 0.1-10.0% SrO, 0-5.0% ZnO, 5.0-20.0% MgO+CaO+SrO+BaO+ZnO, 0-5.0% ZrO₂, and 0-5.0% TiO₂, said substrate being essentially free of alkali metal oxide, having a density not exceeding 2.55 g/cm³, and a chemical resistance so that the glass surface does not change after the glass is immersed for 30 minutes in a buffered hydrofluoric acid solution held at 20° C., said buffered hydrofluoric acid solution comprising, by weight, 38.7% ammonium fluoride and 1.6% hydrofluoric acid. 